Very slow dropping of the boom due to leakages can never be ruled out with absolute certainty when load-holding valves or check valves are used. This is hardly noticeable owing to the large oil volume in the luffing cylinders. If, due to the surrounding area and task, e.g. in narrow street canyons or refineries, the crane cannot be removed during prolonged downtimes, a minor leakage as a result of contamination, for example, causes the boom or part of the boom to drop. This may result in collisions with the surrounding area or tilting of the crane.
It has been known to provide one or more load-holding valves, which check the pressure in the luffing cylinder(s). Smaller construction vehicles, e.g. excavators, are usually provided with mechanical safety devices in order to prevent the boom from lowering due to leakages. U-shaped sleeves placed around the cylinder rods can be seen, for example, at construction vehicle fairs. Such sleeves would be too heavy and too difficult to handle for mobile cranes.
The object of the invention is to prevent accidents caused by very slow unnoticeable changes in the setup condition as a result, for example, of unnoticed lowering of booms or boom extensions, or unnoticed telescopically retracting booms or boom extensions, or unnoticeably retracting outrigger cylinders when the crane operator is not in the cabin and the crane is not in operational use.